Method of inhibiting formation of black spots on tinned copper



Patented Dec. 14, 1943 METHOD OF INHIBITING FORMATION OF BLACK SPOTS ON TINNED COPPER Herbert C. Jennison, deceased,

Conn., by Helen E. Jennison,

field, C onn., Conn., assignors necticut 4 and John C. Bradley,

to The American Brass Company, Waterbury, Conn.,

late of Fairfield, executrix, Fair- Waterbury,

a corporation of Con- No Drawing. Application January 16, 1941, Serial No. 374,700

5 Claims.

This invention relates to a method of inhibiting formation of black spots on tinned copper, and this application is a continuation in part of our prior application, Serial No. 160,778, filed August 25, 1937.

For years the dairy machinery industry has used tinned copper for making milk vats, coolers, etc. Tinned copper, that is copper coated with a thin layer of tin, is liked for these articles because it is of comparatively low cost and can be handled and fabricated more easily than the substitutes, such as enamel ware, stainless steel, rich nickel alloys, etc. There is, however, one particularly serious difiiculty with tinned copper which is known as black spots or tin pest. It is a serious problem in caring for dairy apparatus. This condition is not due to corrosion of the base metal but rather to an allotropic change in the tin which results in disintegration of the-coating. Applicants method prevents or greatly hinders this allotropic change.

It is therefore an object of the present invention to prevent or delay the formation of these black spots, and we have found that this may be done by incorporating a small amount of one or more of certain other metals with the tin. We have found that when a small amount of one or more of the metals of'the group consisting of nickel, cobalt, and iron are deposited with the tin, either simultaneously or successively, the formation of these black spots is materially delayed. It is thought there is a real alloyformed of tin and the other metal or metals at the surface, and that thisalloy inhibits the allotropic change in the tin. Nickel, cobalt and iron have a similar effect but our tests indicate nickel is the most effective in preventing black spots.

For example, we added a cyanide to the regular sodium stannate electrolyte used for electroplating tin upon copper, dissolved nickel or nickel salts in this and then made an electrodeposit which when tested had a material improvement in resistance to formation of black spots as com-v pared with the pure tin coating. One electrolyte contained in solution 1.135 ounces of tin per in combination with tin in the same manner also gallon, .0867 ounce of nickel per gallon and ADA-- ounce of cyanide per gallon. In this case the' nickel and the tin were simultaneously deposited, but we have also found that the formation of the black spots is hindered or delayed if a coat of commercially pure tin is first put onto the copper and then the tin-nickel coat deposited, or we can put on the metals separately and in succession, first the layer of tin, then the other metal or metals, as nickel for example, or if-deprevent or greatly hinder the formation of black spots.

Nor are we confined to the use of pure metals.

In one instance in our tests the tin coating on one sample contained approximately 90% tin, 9% nickel and 1% iron. The point is that when tin is deposited on the copper in combination with certain other metals the black spot formation is materially hindered. We have had as high as 30% nickel present but prefer less because the coating is more ductile. The tin in the coating should greatly predominate with the.

other metal or metals in relatively small amount. Any small amount of the added metal 'assists in preventing black spot and we prefer not to use more than about ten percent as this has been found to be suflicient. v

The coating is put on preferably by codeposition of the metals from a single electroplating bath, but as suggested above we have also accomplishedthe desired end by successive deposits from difierent baths, as for instance from a tin bath for ten minutes, then from a nickel bath for two minutes, and then from a tin bath for two minutes. In another test copper was plated with cobalt and tin. The solution used-was cobalt sulphate in sodium cyanide. To this was added some of the regular sodium stannate plating solution. The plating surface was a little darker than pure tin much as the nickel-tin combination is the darker. 0n testing this sample it was evident that this cobalt-tin deposit was considerably better than pure tin. Analysis of the coating confirmed the presence of an appreciable amount of cobalt in the tin coating but with the tin greatly predominating. a

In :other example'copper was plated with iron and tin. In this case we used ferric sulphate, ammonium oxalate, oxalic acid, stannic chloride, and a solved in water. at about C.

little sodium perborate, all dis- The plating was accomplished The plate was not as good look-.

ing as with the other materials, but when the ting with a small amount of the other metal or metals which have the effect of jiiihibiti'ng'the v allotropic change in the formation of black spots or tin pest.

We do not wish to fuse thetin coating either in. depositing or after it has been"; deposited.

Fusing a tin coating on copper definitely thins the tin coatby making thetin difi usetinto the copper and thus really result in a bronze coating of mostly copper instead of a tin coating' or coat- 10 ing mostly tin. In other words fusing ofthef tin coating is undesirable in our method for inhibiting black spots as our experiments have" indicatedit tends to reduce the resistance of the coating to this disease.

Having thus set forth the nature of our invention what we claim is:

1. A method of inhibiting formation of black spots on tinned copper which consists in electro spots on tinriedcopper which-consists in electrodepositing on the copper a coating consisting of tin and an amount of metal of the group consist: ing of nickel, cobalt and ironsuflicient to'in-' hib'it-black spot. formation, and with the resultant coating essentially tin and of at 'least about 90 per cent of this metal.

-3. A method of inhibiting formation of black spots on tinned copper which consists in electrodepositing on the tinned surface a coating consisting; ofrtin-iand an amount of metal of the group consistingof nickel, cobalt andiron sufiicient to inhibit black spot formation, and with the resultant coating essentially tin and-of at leastaboutilo per cent of this metal.

4. 'A method-of inhibiting formation of black spotson tinned copper which consists in simultaneously'electrodepositing on the copper from a single electroplating bath as a coating tin and an amount of other metal from the group consisting of 5 nickel, cobalt and iron sufficient to inhibit 'black" spot formation, and with the resultant coatingfessentially'tin and of at least" about 90 per cent -of this' metal.

.5.-'An article of manufacture for use in handling foodand beveragefproducts, including fabricated vessels, containers or structures made at least partly of 'copper andamelectrodeposited coating on the-copper consisting es'sentially 'of tin in amount at'least about90 percent a'nd an amount of metal (of the group consisting of nickel, cobalt and iron sufiicient .to inhibit black spot formatiomsaid coating beingronthat suriaceto ,be-'us'edin'- contact .with the food and beverage products whereby-the formationof black spots on this c'oatedsurface is-inhibited.

/ HELEN JENNISON; Ex'ecutria: of Herbert c'gJenm'son, Deceased.

- JOHN o. BRADLEY."- 

